As a hydrometallurgical method whereby valuable metals, such as nickel and cobalt, are recovered from low-grade nickel oxide ores, typified by limonite ores, etc., a high-temperature pressurized sulfuric acid leaching method, which is high-pressure acid leaching (HPAL) using sulfuric acid, is known.
As shown in FIG. 1, hydrometallurgy for obtaining a nickel-cobalt mixed sulfide from a nickel oxide ore comprises a pretreatment step (1), a high-temperature pressurized sulfuric acid leaching step (2), a solid-liquid separation step (3), a neutralization step (4), a dezincification step (5), a sulfurization step (6), and a detoxification step (7) (see, for example, PTL 1).
In the pretreatment step (1), a nickel oxide ore is ground and classified to prepare an ore slurry. In the high-temperature pressurized sulfuric acid leaching step (2), sulfuric acid is added to the ore slurry obtained in the pretreatment step (1), and high-temperature pressurized acid leaching is performed by stirring the mixture at 220 to 280° C. to thereby obtain a leached slurry. In the solid-liquid separation step (3), the leached slurry obtained in the high-temperature pressurized sulfuric acid leaching step (2) is subjected to solid-liquid separation to obtain a leachate (a crude nickel sulfate aqueous solution) containing nickel and cobalt as well as impurity elements, and a leach residue.
In the neutralization step (4), the crude nickel sulfate aqueous solution obtained in the solid-liquid separation step (3) is neutralized, and a neutralized precipitate containing impurity elements is separated to obtain a neutralized solution containing nickel and cobalt as well as zinc. In the dezincification step (5), hydrogen sulfide gas is added to the neutralized solution obtained in the neutralization step (4), and zinc is precipitated and removed as zinc sulfide to obtain a nickel recovery mother liquor containing nickel and cobalt. In the sulfurization step (6), hydrogen sulfide gas is added to the nickel recovery mother liquor obtained in the dezincification step (5) to obtain a nickel-cobalt mixed sulfide and a nickel barren solution. In the detoxification step (7), the leach residue generated in the solid-liquid separation step (3) and the nickel barren solution generated in the sulfurization step (6) are detoxified.
The solubility of sulfide in the pH range in the above hydrometallurgy is lower in zinc than in nickel and cobalt, and zinc is more likely to be precipitated as sulfide even at the same pH. In the dezincification step (5), taking advantage of this solubility difference, zinc, which is an impurity, is selectively precipitated and removed by controlling the amount of a sulfurizing agent added and adjusting the pH.
Here, the pH adjustment is performed in the neutralization step (4), which is the preceding step of the dezincification step (5). In the neutralization step (4), the pH of the neutralized solution (dezincification step start solution) is adjusted by controlling the amount of the neutralizing agent added based on the value measured by a pH meter.
However, the pH meter is likely to be affected by changes in the liquid temperature, and neutralized precipitates adhere to the surface of the pH meter due to a long-term operation. Because of these influences, values measured by the pH meter change, thereby causing insufficient addition or excessive addition of the neutralizing agent.
If the pH of the neutralized solution decreases due to insufficient addition of the neutralizing agent, there is a problem that the removal efficiency of zinc in the dezincification step (5) is reduced. On the other hand, if the pH of the neutralized solution increases due to excessive addition of the neutralizing agent, fine particles of hydroxide and plaster floating in the neutralized solution increase. Accordingly, there is a problem that in the dezincification step (5), the filter cloth of the solid-liquid separator that performs solid-liquid separation to separate zinc sulfide and a nickel recovery mother liquor is clogged, thereby leading to a reduction in the efficiency of solid-liquid separation.